In the manufacturing of corrugated paperboard, a single facer apparatus is used to flute the medium web, to apply adhesive (e.g. starch adhesives) to the flute tips on one of the faces of the medium web, and to bring a liner web into contact with glue lines on the flute tips of the medium web with the application of heat downstream to form the initial bond. Many manufacturers in the art use high-pressure (e.g. 20,000 lbs. of force) at the bonding nip where the liner web is introduced to the flute tips on the medium web. Others use less pressure at the bonding nip. The assignee of this application produces a single facer in which the liner roll introducing the liner web to the fluted medium web is spaced apart from the fluted surface of the bonding roll. The present invention involves the use of steam injection at the convergence of the webs to improve the formation of adhesive bonds between the liner web and the flute tips on the medium web, especially at relatively high production speeds (e.g. 1300 feet per minute). It is particularly useful on single facers that do not have a high pressure bonding nip.
When applied to the flute tips on the medium web, the starch adhesive consists of a slurry of starch granules in water (e.g. an aqueous-based starch adhesive). In order to achieve optimum bond strength, it is desirable that the water and starch granule slurry first gelatinize into a viscous gel. In order to gelatinize the starch adhesive, the adhesive slurry must be heated to a temperature of at least 150.degree. F. for a sufficient amount of time. It is therefore known in the art to preheat the liner web to promote gelatinization of the adhesive, preferably to about 180.degree. to 212.degree. F. The viscous gel does not have significant bond strength until dehydration occurs. However, if dehydration occurs prematurely before gelatinization, the resulting bond will be poor.
It is important that the initial green bonds formed on the single facer between the liner web and the flute tips of the medium web have sufficient strength to withstand further processing downstream in the corrugator. Although green bond formation begins immediately at about 200.degree. F., substantial dehydration of the glue line and adjoining web surfaces must occur before adequate green bond strength is realized. The medium web is heated on the bonding roll preferably to about 215.degree. F. to facilitate dehydration and formation of the green bond. In order to achieve sufficient heat transfer, the bonding roll surface temperature should be at about 375.degree. F. or more. As more heat is added to evaporate moisture, the bond strength continues to increase. A fully cured bond is finally obtained when most of the water is evaporated (e.g. approximately 80%).
As production speed increases, exposure times for gelatinization and green bond formation are proportionally reduced at the same time that the level of mechanical stress imposed on the freshly bonded corrugated web are increased. Heretofore, it has been difficult to increase production speeds above, for example, 1,000 feet per minute without sacrificing bond quality.